1. Field of the Invention
The present invention relates to a film depositing apparatus and a process for preparing a layered structure including an oxide superconductor thin film, and more specifically to an improved film depositing apparatus particularly for preparing a layered structure including an oxide superconductor thin film and a dielectric thin film and/or an insulator thin film, which has a clear interface with negligible interface states, high crystallinity and excellent properties.
2. Description of Related Art
Oxide superconductors have been found to have higher critical temperatures than those of metal superconductors, and therefore considered to have good possibility of practical application. For example, Y--Ba--Cu--O type oxide superconductor has a critical temperature higher than 80 K. and it is reported that Bi--Sr--Ca--Cu--O type oxide superconductor and Tl--Ba--Ca--Cu--O type oxide superconductor have critical temperatures higher than 100 K.
In case of applying the oxide superconductor to superconducting electronics including superconducting devices and superconducting integrated circuits, the oxide superconductor has to be used in the form of a thin film having a thickness of a few nanometers to some hundreds micrometers. It is considered to be preferable to utilize various deposition methods, such as sputtering methods, laser ablation methods and reactive co-evaporation methods for forming oxide superconductor thin films. In particular, it is possible to deposit an oxide superconductor thin film atomic layer by atomic layer through utilizing a reactive co-evaporation method. Additionally, in-situ observation during and between depositing thin film is possible so that a high quality oxide superconductor thin film can be obtained by the reactive co-evaporation method.
In order to deposit an oxide superconductor thin film on a substrate by the reactive co-evaporation method, constituent elements of the oxide superconductor excluding oxygen are supplied as molecular beams towards the substrate by using Knudsen's cell (abbreviated to K cell hereinafter) type molecular beam sources. In addition, an oxidizing gas such as O.sub.2 including O.sub.3, NO.sub.2 or N.sub.2 O is supplied near the substrate so that the molecular beams are oxidized so as to form the oxide superconductor thin film on the substrate. It is also possible to deposit high quality thin films of ferroelectrics such as SrTiO.sub.3 and of nonsuperconducting oxide such as Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y. Furthermore, by switching molecular beam sources, it is possible to deposit thin films of different materials or compositions successively so as to form a layered structure.
In U.S. patent application Ser. No. 07/987,756, Takao Nakamura discloses a film deposition apparatus suitable for depositing oxide thin films by the reactive co-evaporation method. This film deposition apparatus includes a vacuum chamber provided with a main evacuating apparatus, at least one K cell or at least one electron beam gun provided at a bottom of the vacuum chamber, and a sample holder provided at a top of the vacuum chamber for holding a substrate on which a film is to be deposited. The sample holder is associated with a heater for heating the substrate. In addition, the vacuum chamber is also provided with a liquid nitrogen shroud for forming a cold trap around an evaporation source of the K cell or electron beam gun, and a RHEED (Reflecting High Energy Electron Diffraction) device for evaluating a depositing thin film. In front of the substrate attached to the sample holder, a shutter is located for controlling a deposition time during the deposition process. The K cell and the electron beam gun are also provided with an operatable shutter.
In addition, a gas supplying apparatus is provided so as to introduce an oxidizing gas such as O.sub.2, O.sub.3, NO.sub.2, N.sub.2 O, etc. in vicinity of the substrate attached to the sample holder, so that the oxidizing gas can be supplied to form an oxygen-enriched atmosphere in the vicinity of the substrate in order to oxidize molecular beams incoming from the molecular beam source in the course of the film deposition.
Furthermore, the film deposition apparatus additionally includes a partitioning plate for dividing the vacuum chamber into a first sub-chamber which is constituted of a lower portion of the vacuum chamber defined below the partitioning plate and which is coupled to the K cell, the electron beam gun and the main evacuating apparatus, and a second sub-chamber which is constituted of an upper portion of the vacuum chamber defined above the partitioning plate and in which sample holder is located. The partitioning plate includes a through opening formed at a center thereof. The position of the opening is determined to ensure that a beam emitted from K cell and the electron beam gun toward the substrate is not obstructed by the partitioning plate. In addition, the size of the opening is determined to enable restricted molecular flows, particularly of oxygen gas, from the second sub-chamber to the first sub-chamber so that a pressure difference can be created between the first sub-chamber and the second sub-chamber even if the opening is open. Therefore, the partitioning plate having the through opening constitutes a vacuum conductance.
A gate valve is provided on the partitioning plate for hermetically closing the opening of the partitioning plate, so as to completely shut off the molecular flows between the first sub-chamber and the second sub-chamber when the gate valve is closed. An opening and closing of this gate valve is controlled from the outside of the film deposition apparatus.
In addition, an auxiliary evacuating apparatus is attached to the second sub-chamber for evacuating the second sub-chamber to an ultra-high vacuum even if the gate valve is closed.
By using the above conventional film deposition apparatus, a high quality single oxide thin film of high crystallinity with excellent properties can be deposited. However, it is difficult to form a layered structure composed of a sharp and clean interface and thin films of different materials or compositions with excellent properties.
In the above film deposition apparatus, a composition of a deposited thin film is controlled by ratios of intensity of the molecular beams and the intensity of the molecular beams is controlled by temperatures of crucibles of the K cell type molecular beam sources. Therefore, accurate temperature controls of the crucibles are required when the or molecular beam sources are switched so as to deposit a different thin film.
Amounts of materials left in the crucibles and atmosphere around the molecular beam sources effects on the intensity of the molecular beams and temperature distributions of around the out lets of the crucibles are influenced by an operation of shutters. Therefore, the temperatures of the crucibles should be stabled before starting the deposition of a thin film. It takes long time to stabilize the temperatures of the crucibles so that process for forming a layered structure is interrupted when the molecular beam sources are switched so as to deposit a different thin film.
During the interruption, a lower thin film just deposited is maintained at the depositing temperature so that contaminants in the chamber may stick to a surface of the lower thin film or oxygen within the lower thin film may escape so as to diffuse into the chamber of ultra high vacuum.
In order to prevent the above phenomenon, it is proposed to stop heating the substrate during the interruption so as to lower the temperature of a lower thin film. However, in this case, a lower thin film is repeatedly heated and cooled so as to subject large thermal hysteresis which causes distortions and clacks of the thin film.